This invention relates to making armatures for dynamo-electric machines such as motors and generators. Although the invention will be described primarily in the context of its application to electric motor armatures, it will be appreciated that it is equally applicable to rotating rotors in general which are wound with wire for conducting electric current. For convenience, all such rotors are referred to herein as armatures. Also, although the convention will be described primarily in the context of flyer type armature coil winders, it will be understood that the invention is equally applicable to winders that employ other types of coil wire dispensing members such as the apparatus shown in commonly assigned, U.S. Pat. No. 5,484,114.
As shown in FIG. 1, finished armatures 10 wound with wire 12 in coil receiving slots 14 of a lamination stack 16 need to be precisely balanced prior to their final operational use. This avoids mechanical malfunctioning, and also guarantees the integrity of the armature, together with that of other components which are assembled in the environment where the final operational use occurs.
It is common practice to use automatic balancing machines at the end of an armature production line to determine how unbalanced the armature has become during the processing stages and to correct for this unbalanced condition by adding or removing masses on certain parts of the finished armature. The most common technique for automatic balancing of armatures removes masses by milling one or more grooves in the outer circumferential surface of the armature stack 16.
An unbalanced armature (requiring balancing as described above) may be the result of an unbalanced stack 16, shaft 18, or commutator 19, and also may be the result of the overall disposition of the masses of these components as a result of the operations required to assemble them to form the armature. An unbalanced armature can also result from the operational steps required to wind the coils of wire 12 in the slots 14 of stack 16. Although the disposition of the coils (and their number of turns) around the armature is theoretically correct for avoiding an unbalanced condition, practice has shown that the winding process can unbalance the armature.
In order to reduce the need for or the required extent of a final balancing step in the manufacture of armatures, it is an object of this invention to balance the armature during winding.
Formation of armature coils requires simultaneous winding of two wires in two pairs of slots which are symmetrically opposite one another as shown in accompanying FIG. 2. For example, coil 20 is wound in slot pair 22, 23 symmetrically opposite to coil 21 which is simultaneously wound in slot pair 24, 25.
One of the fundamental production specifications for winding armatures usually requires winding symmetrically opposite pairs of slots (such as those referenced above) with the same number of turns of wire. As has been mentioned, this creates a theoretical basis for balancing the armature, although, as will be more fully described below, in practice during winding various factors can cause an unbalanced condition.
Armatures of the type shown in accompanying FIG. 1 are frequently wound with a flyer type winder, although other types of winders e.g., those shown in above-mentioned U.S. Pat. No. 5,484,114 which is hereby incorporated by reference herein)) are also known and are subject to the same problems and solutions discussed herein. As shown in accompanying FIGS. 3 and 4, the typical flyer type winder includes two opposite flyers 30, 31 which can rotate around respective axes 32, 33 so that each of them dispenses an associated wire 34, 35 coming from a wire spool 36, 37 into a respective pair of coil receiving slots 38, 39 and 40, 41 aligned with prepositioned winding forms 42, 43. The winding forms are required to guide each wire into the coil receiving slots as the wire leaves the associated flyer. The wires required to wind the coils, prior to reaching the flyers from the wire spools, pass through respective tensioner devices 46, 47 which are supposed to guarantee that predetermined tensions are maintained on the wires during the various operations required to wind and form the leads of the armature. The two flyers 30 and 31 rotate at the same time so that each of them forms a coil in respective pairs of slots which are symmetrically disposed on opposite sides of a central transverse axis 80 of the armature. Flyers 30 and 31 are driven by independent motors 44, 45, which are controlled to rotate in unison so that both flyers reach, as precisely as possible, similar predetermined angular positions in time. In particular, the two flyers start and terminate rotation at the same time so that both coils are wound simultaneously with the same number of turns.
At any given instant of time during winding, a difference between the tension of the wires being wound by their respective flyers can result in different elongation of the wires. In a comparison between the two flyers, which are winding opposite coils at the same time, this leads to supplying in certain instances different masses of wire into symmetrically opposite pairs of slots of the armature (such as those shown in FIG. 4). This has an unbalancing effect on the armature. In addition, a coil wound with higher tension will have more compact turns, which influences the radial disposition of its mass (e.g., in relation to the central longitudinal axis 82 of the armature). This also contributes to the formation of unbalanced armatures if variations of this type exist between the opposite coils being formed at the same time by the two flyers.
The foregoing considerations can be further illustrated with the aid of accompanying FIG. 5, in which certain features are somewhat exaggerated. The wires relating to a few coil turns for respective opposite coils 20, 21 are shown. The turns of coil 21 are wound with higher tension, which subjects the wire to a greater amount of elongation for the same number of turns. This causes coil 21 to have less wire mass and to be more compact toward the central longitudinal axis 82 of the armature than coil 20. It should be appreciated that the formation of the overall coils of the armature requires a progressive build-up of wire turns and also of different coils. Later-wound turns and coils surmount earlier-wound turns and coils so that the later-wound material is farther away from central axis 82. As a result of this overlying or overlapping, the presence of an internal coil which is less compact tends to amplify the lack of balance because it also affects the mass disposition of successive coils which will be positioned farther away from the central longitudinal axis 82 of the armature.
One approach for balancing armatures is described in commonly assigned U.S. Pat. No. 5,383,619. As described in U. S. Pat. No. 5,393,619, one way in which to balance an armature involves measuring the amount of wire leaving each of the two wire spools during winding using encoders to determine the velocity or length of the wire. This measurement reveals approximately how much wire is being wound onto the armature by each of the two flyers. If it is determined that more wire is being wound onto the armature by one of the flyers than the other, than the rate at which wire is delivered to each of the flyers can be adjusted.
As described in the above-mentioned U.S. Pat. No. 5,383,619, the wire that is fed to the flyers is tensioned using wire tensioners such as hysteresis brakes. The brakes are adjusted to balance the amount of wire that is being fed to each of the flyers by varying the tension each brake applies to the wire. For example, the retarding force on the wire being wound onto one coil could be increased, so that subsequently, less wire is wound onto that coil.
With this arrangement, rotating encoders 60 for measuring wire consumption are typically placed immediately adjacent to the spool 36, as shown in FIG. 6, which is a reproduction of FIG. 6 of the above-mentioned U.S. Pat. No. 5,383,619. For each flyer, tensioning device 46 is typically placed downstream of encoder 60. After tensioning device 46, wire 34 runs over a pulley wheel 68 of a spring biased dancer arm 69 prior to reaching flyer 30.
The dancer arm 69 is primarily required during operations in which the flyer undergoes abrupt changes in rotation direction and speed. In these situations the dancer arm accommodates any abrupt tightening or loosening of the wire that may occur by resiliently pivoting about axle 70 in the appropriate direction 71. Although the arrangement shown in FIG. 6 is satisfactory for balancing armatures in many situations, such an arrangement does not account for the changes in the length of the wire between tensioning device 46 and flyer 30 due to motions of the dancer arm 69 that might occur under extreme tensioning conditions. As a result, at any given time, the wire consumption measured by encoder 60 may not represent the amount of wire that has actually been wound onto the armature as accurately as might be desired.
Another arrangement for balancing armatures during coil winding is shown in FIG. 7, which is a reproduction of FIG. 11 of the above-mentioned U.S. Pat No. 5,383,619. Using this approach, wire tension sensors 120 and 121 are provided to measure the tension of the wire being wound onto the armature by flyers 30 and 31. Although the arrangement of FIG. 7 can be used to balance armatures by maintaining the wire tensions measured by sensors 120 and 121 at the same level, tension sensors may not always be as precise as desired when operating at extremely high speeds.
One of the components of conventional winding machines is the wire tensioner. Typically, using the arrangement described in the above mentioned U.S. Pat. No 5,383,619, two hysteresis brakes are used to tension the wires as they are fed to respective flyers. Conventional hysteresis brakes contain a stationary stator. A rotor is mounted within the stator for rotational motion. Current is supplied to the field coils of the stator to produce a retarding torque between the stator and the rotating rotor. A pulley attached to the rotor axis applies this retarding torque to the wire. The magnitude of the retarding torque is controlled by varying the current to the stator.
To decrease the wire tension, for example, when it is desired to form lead connections to the commutator of an armature following coil formation, the control current is lowered. However, conventional hysteresis brakes suffer from an effect known as "cogging," in which the tension applied by the brake remains high even after the control current has been lowered. The tension remains at this high level until the rotor has been forcibly moved by the tension of the wire through an angular distance equal to the distance between successive poles. Because the torque remains high, the lead connections are exposed to a larger wire tension than is desired, which can prevent the lead connections from being formed properly.
In view of the foregoing, it is an object of the present invention to provide improved methods and apparatus for balancing armatures during the process of winding.
It is another object of this invention to provide an improved hysteresis brake for use as a wire tensioner that overcomes the effects of cogging.